292 



DISEASES OF ECONOMIC PLANTS 



gradually extend in all directions, involving the whole 

 root. Older spots, 2-5 cm. across, often break or crack 

 irregularly near the center. This 

 decay differs from the soft rot in 

 that the spots are dry and hard 

 and that it is present upon the 

 roots before digging. With the 

 hand lens very small, hairlike, 

 black structures, 1 mm. high, are 

 seen studding the centers of the 

 diseased areas. These are the 

 beaks of the pycnidia of the causal 

 fungus. 



Upon young sprouts before they 

 are set out the disease causes black, 

 dead patches, especially at the base 

 of the shoot or even upon the young 

 leaves. The wood of the stem may 

 be browned. Such sprouts if set 

 result in enfeebled plants with dis- 

 eased roots. 



The fungus throws myriads of 

 spores into the soil. This disease 

 thus falls under the general class 

 of soil diseases. 



In addition to the suggestions 

 given on page 63, especial precau- 

 tion should be exercised to avoid 

 sets already affected. Badly af- 

 fected shoots are easy of recognition, but the closest 

 scrutiny is needed to cull out those only slightly affected. 



.t 



127. — Black rot of sweet 

 potato. Original. 



